1. Field of the Invention
The present invention relates to SAW based sensors for measuring both torque and temperature.
2. The Prior Art
Applicant's own earlier British Patent no. 2381069 discloses a SAW sensor for the contactless measurement of torque on a rotating shaft comprising two SAW devices mounted on a single substrate made of Y+34° cut quartz, one of them oriented at +45° to X axis of the substrate and another one oriented at −45° to the X axis. The difference between the signals obtained from the two SAW devices (the resonant frequency difference FM=f1−f2 if the two devices are resonators or the phase delay difference if the two devices are reflective delay lines) is then proportional to torque, and differential measurement allows partial compensation of aging effect and influence of temperature and the rotational error introduced by an RF rotary coupler. The use of the Y+34° cut of quartz and the SAW devices being oriented at 45° to the X axis gives a high value of torque sensitivity, reduced variation of torque sensitivity with temperature, and reduced variation of resonant frequencies with temperature.
This prior art system therefore simplifies, to a certain extent, temperature compensation of the torque reading but does not completely eliminate the need for the compensation. Indeed, typical variation of the sensitivity of the torque sensor attached to a steel shaft is around 1-2% within the temperature range from −30° to +90° C. Moreover the frequency difference FM at zero torque (zero offset F0) also varies with temperature due to the fact that the two SAW devices are not absolutely identical by about 8-10% of full scale. As a result, in order to achieve a high accuracy of torque measurements we need to measure the temperature as well and then use calibration information to perform compensation. Temperature measurements are typically therefore done by an external semiconductor, thermoresistor or thermocouple sensor attached to a place in a close proximity to the torque sensor. However these devices must inevitably be spaced apart from the SAW substrate, thereby imposing a limit on the accuracy of the temperature reading for the substrate which can be taken.
GB 2386684 discloses a SAW based pressure sensor having 3 SAW devices mounted on a single substrate so as to enable both pressure and temperature readings to be taken. One of the SAW resonators, PSAW, is mounted on a portion of the substrate which is subjected to strain variations due to changes in pressure whilst the other two are mounted on unstrained regions of the substrate. By using three resonators, two frequency differentials can be calculated: Fp—the difference between the resonant frequencies of PSAW and one of the unstrained SAWs that is parallel to PSAW (T1SAW), which figure depends predominantly on pressure only (temperature compensation of Fp is achieved by means of differential measurement). The third resonator (T2SAW) is at an angle to T1SAW and PSAW so its temperature characteristic differs from that of T1SAW due to the substrate anisotropy. As a result, the difference Ft of resonant frequencies of T1SAW and T2SAW depends only on temperature thus allowing easy calculation of temperature from the measured value of Ft.